A typical metal halide discharge lamp 10 is illustrated in FIG. 1, and includes a body 11 and a first leg 12 and a second leg 13 integrally attached to the body 11. Each leg 12 and 13 extends from an opposing side of the body 11. The legs 12 and 13 and body 11 are usually fabricated from a quartz-material or an alumina based ceramic material (e.g., polycrystalline alumina, sapphire, or yttrium aluminum garnet). A first electrode 15 and second electrode 16 extend through the first leg 12 and second leg 13 respectively and terminate in a chamber 14 formed in the body 11 of the lamp 10. The tips 15A and 16A of the electrodes are spaced apart a determined distance within the chamber 14, ranging from about 1 mm to about 20 mm forming an arc region between the electrode tips 15A and 16A. The volume of the chamber 14 is typically within the range of about 0.01 cc to about 3 cc. The chamber 14 is sealed under pressure at the ends of the legs 12 and 13 distal the chamber.
Before the chamber 14 is sealed, a composition including an inert gas, a metal halide dose and mercury is injected and sealed, under controlled atmosphere, in the chamber of the discharge lamp. The metal halide dose is typically a combination of metal halides such as sodium iodide and scandium iodide or sodium iodides, thallium iodide, dysprosium iodide, holmium iodide and thulium iodide. The metal halides serve as light emitting elements. While mercury contributes slightly to the emitted spectrum of a discharge lamp in the blue range, it mainly serves to increase the electrical resistance in the arc region in order to raise the voltage to a desired value. Raising the voltage to a desired value has two effects: 1) the lamp operating current can be maintained at a low value to minimize electrode erosion for better lumen maintenance and lamp life; and, 2) minimizing end-losses for better lamp efficiency. A desired operating voltage for a high intensity discharge lamp is typically from 70V to 150V so the current can be maintained from about 0.2 amps to about 3.5 amps depending on the type of lamp and a desired power.
When power is supplied to the electrodes, and an electric arc strikes between the electrode tips 15A and 16A, creating a plasma discharge within the chamber 14. Initially an arc discharge is created by the rare gas (typically argon or xenon) reaching a temperature of about 7000K. The arc discharge heats the chamber 14 raising its temperature to about 1000° K or higher. Then the mercury and metal halide dose start evaporating. After this warm-up phase, the lamp reaches a steady state of operation, where the plasma discharge becomes a mixture of rare gas atoms (argon or xenon), Hg atoms and ions, metal atoms and molecules coming from the metal halide dose as well as their ions and the electrons. The temperature of the plasma discharge may range typically from about 1000° K to about 6000° K.
The lamp voltage depends strongly on the electrical conductivity of the gas mixture forming the arc. In typical HID lamps, mercury serves as a buffer gas by maintaining a certain desired lamp operating voltage. Mercury may achieve the desired voltage because of its relatively low electrical conductivity, which is the function of several parameters including atom density (or vapor pressure), electron density (or ionization energy) and electron-atom momentum transfer cross-section for the so-called buffer gas.
Mercury, as a buffer gas, has a high enough electron-atom momentum transfer cross-section and high enough vapor pressure to provide a sufficient electrical resistance at the arc region and therefore a desired lamp voltage. The collision between electrons and the metal halide compounds causes excitation of the metal atoms, which release photon energy in the form of light within the visible spectrum.
Despite the effectiveness of mercury, there are disadvantages to using this metal. Most notably, mercury is very toxic and raises health and environmental concerns. Laws and regulations have been adopted and/or proposed throughout the world limiting or, in some cases eliminating the use of mercury in all products. Accordingly, efforts are being made to replace mercury with other elements or compounds that have properties similar to mercury for purposes of generating light in a high intensity discharge lamp.
Zinc iodide has been disclosed as a substitute for mercury in the presence of metal halide additives sodium iodide (NaI) and scandium iodide (ScI3) in a quartz lamp. However, scandium is aggressive toward and reactive with alumina-based ceramics, which is the envelope material to be used in the next generation automotive headlamps.
Rare earth metal halides, such as dysprosium iodide and neodymium iodide have been disclosed as a substitute for scandium iodide (ScI3) in combination with a second metal halide that is substituted for mercury in a quartz lamp. The second metal halides include aluminum iodide, iron iodide, zinc iodide, antimony iodide, manganese iodide, chromium iodide, gallium iodide, beryllium iodide and titanium iodide.
With respect to the subject inventions various combinations of metal halides, including but not limited to zinc iodide, as a substitute for mercury, in combination with one or more rare earth metal halides, sodium iodide and thallium iodide as light emitting additives, were combined and tested in a ceramic metal halide lamp. The performance of these compounds were compared to metal halide ceramic lamps having a composition of mercury combined with the same combinations of the rare earth metal halides, sodium iodide and thallium iodide as the light emitting elements. Theoretical calculations supported by experimental tests have shown that mercury substitute metal halides disassociate into metal atoms and free iodine atoms within the arc region causing a high pressure of free iodine atoms. Iodine is known to be very electronegative. That is free electrons within the arc region attach relatively easily to the iodine atoms creating negative ions of iodine. This effect causes a significant reduction in the electrons density within the arc region. Furthermore, the iodine reacts with the rare earth metal forming stable compounds, i.e. dysprosium iodide, which causes the reduction in the density of rare earth metal atoms (light emitting species). The reduction of both electron density and light emitting species atoms (rare earth) caused by the high-pressure of free iodine affect directly in a negative way the lamp performance by reducing the amount of radiated power in the visible range (lamp lumens)
The pressure of the iodine and iodine negative ions in ZnI2 dosed lamp is almost one order of magnitude greater than in the mercury-dosed lamps. This means that the electron density in the arc region as well as the light emitting atom densities are significantly lower in a ZnI2 dosed lamp than in mercury lamp for instance. The net effect is reduced lumens because the electrons and the light emitting atoms are responsible for the creation of the excited states of light emitting metal atoms.